Alkylresorcinols are known to have a melanin production inhibitory effect and an antimicrobial effect, and are incorporated into cosmetics for the purpose of skin whitening and antimicrobial protection (e.g., see Patent References 1 and 2). In addition to the above-mentioned alkylresorcinols, there are known L-ascorbic acid and L-ascorbic acid derivatives as substances having a skin-whitening effect; and there have been proposed external preparations for skin containing an alkylresorcinol and an L-ascorbic acid (=L-ascorbic acid, L-ascorbic acid derivative). (e.g., see Patent References 3 and 4). Alkylresorcinols and L-ascorbic acids each have a problem in that they tend to become discolored due to temperature and light; however, sufficient investigations have not as yet been made for the technique of effectively preventing the discoloration of alkylresorcinols and L-ascorbic acids.